Firstly, hash_file() is expecting a file to already exist and you're trying use that method before the file gets uploaded; that's why your code failed and threw you that error.

What you need to do is to see if that file exists and then hash it.

If this is really want you want to do, then you can base yourself on the following and remember to store the renamed file while retaining its original extension; there are links at the end of the answer.

Note: As I mentioned in comments, you need to hash the file and not the whole destination folder and the file. That would be impossible to retrieve.

Echo the variable for what was assigned to hash_file(). You will also get your hash name (only) shown minus its extension.

Check for errors and make sure the folder has been granted proper permissions.

<?php 

// check for errors
ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);

$file_name = $_FILES['fileToUpload']['name'];
$sent_file = move_uploaded_file($_FILES["fileToUpload"]["tmp_name"], $target_file);

 if (file_exists("images/uploads/" . $_FILES["fileToUpload"]["name"]))
  {
  echo $the_file = $_FILES["fileToUpload"]["name"] . " exists.";
  
    // its new location and hashing the filename only.
   $var = hash_file('sha256', $the_file );
   echo $var;
   
   // store your renamed file here in your database
   // using the assigned $var variable.
  
  }

Also check for errors on the query with mysqli_error($conn).

However, you're going to end up with problems here to show that image, since now and for example in using "file.jpg" will produce the following hash:

cf80cd8aed482d5d1527d7dc72fceff84e6326592848447d2dc0b0e87dfc9a90

I don't know how you plan on showing the image(s), but it will no longer keep the .jpg extension.

In order to retain the image's file extension, you basically need to rename the uploaded file(s).

Here are a few good references on Stack (that I've had success with in the past) that you can look at and implement it in your code. :

• How to rename uploaded file before saving it into a directory?
• Rename uploaded file (php)

There is indeed no better way to learn, IMHO.

Edit:

This is an excerpt from a script I wrote recently. Base yourself on the following.

Note: You shouldn't use hashing methods such as anything from the SHA family or MD5 as the file name, since those produce the same hash and has no uniqueness to them.

Important: If people upload from a mobile device, most of them have "image.jpg" as the default name, so it needs to be renamed and given a unique method.

Using the date and time is one way. You can also add uniqid() to it by assigning a variable to it and append to the new file name, or a combination of MD5 and uniqid() is a good bet.

You will need to do a few modifications to it of course. The $year variable is something I used but you can get rid of those instances and replace them with your own.

$year = date("Y");
$pdf_file = $_FILES['fileToUpload']["name"];

$uploaded_date = date("Y-m-d_h-i-s_A"); // this could be another unique method.

$target_dir = "../upload_folder/" . $year . "/";

$ext = explode('.',$_FILES['fileToUpload']['name']);
$extension = $ext[1];

$newname = $ext[0].'_'.$uploaded_date;

$full_local_path = $target_dir.$newname.'.'.$extension;
$new_full_name = $newname.'.'.$extension;

    if (move_uploaded_file($_FILES["fileToUpload"]["tmp_name"], $full_local_path)) {
        echo "The file ". $newname . " has been uploaded.";
        echo "<hr>";

$file_link = "/upload_folder/$year/$new_full_name";

    // other code such as saving to a database...

    }